Add fields to platform library, implement them on runtime dart.

tests/lib/platform/path_separator_test.dart

 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-// dart2js used to fail this test, by inferring that `a.last()`
-// returns the element type of the `a` list.
+import "dart:platform" as platform;
+
+import "package:unittest/unittest.dart";
 
 main() {
-  var a = [() => 123];
-  if (a.last() is! num) {
-    throw 'Test failed';
+  if (platform.isWindows) {
+    expect(platform.pathSeparator, '\\');
+  } else {
+    expect(platform.pathSeparator, '/');
   }
 }